The longtime agent, removed from the special counsel's Trump-Russia investigation when the messages were discovered, had defended himself in a combative House hearing. His lawyer says the firing was due to political pressure.
The odds are against anyone ever answering for recent "inadvertent" deletions of electronic data at the two agencies.
The bureau says a technical system glitch caused messages between December 2016 and May 2017 not to have been properly stored.
The surveillance was uncovered after a protester filed a Freedom of Information request about demonstrations in 2014 and 2015.
Leaks reveal more insight into the NSA and GCHQ and the collection of nearly 200 million text messages a day.
New York Times reporter Scott Shane discusses new ways the NSA is known to be "an electronic omnivore of staggering capabilities, eavesdropping and hacking its way around the world to strip governments and other targets of their secrets, all the while enforcing the utmost secrecy about its own operations."
There's a new kind of addiction out there, to which many of us are currently vulnerable, and from which some of us may be suffering right this moment: According to one Dr. Jerald Block, writing in the American Journal of Psychiatry, excessive e-mailing and text messaging could be a form of mental illness.